The invention relates to medical devices, and in particular, to medical devices used to guide diagnosis, monitoring and/or treatment of respiratory conditions.
Every day, patients with difficulty breathing seek medical help. In such cases, the patients may complain of shortness of breath, but may have no idea as to the cause of the condition. Many cases of shortness of breath fall into two general categories of respiratory disorders.
One category of respiratory disorder that may cause shortness of breath is obstructive lung disease. A patient with obstructive lung disease suffers from a narrowing of the airways leading to the alveoli in the lungs. This narrowing, often caused by inflammatory reactions, results in a reduction of the patient""s ability to ventilate the alveoli, because the narrowed airways reduce the maximum velocity of flow through the airways. Chronic obstructive pulmonary diseases such as asthma, bronchitis and emphysema, are some of the disorders that can cause narrowing of the airway.
A second category of respiratory disorder that may cause shortness of breath is restrictive lung disease. Restrictive lung disease is characterized by a reduction of the overall gas-exchange area in the lungs. A restrictive lung disease may be temporary, such as a short-term filling of the alveoli with fluid, or more long-lasting, such as fibrosis that prevents the alveoli from expanding during inhalation. A restrictive lung disease may also be caused by congestive heart failure leading to pulmonary edema.
When a patient complains of difficulty breathing, it is difficult for health care professionals to rapidly determine whether the problem is due to obstructive or restrictive origins. The symptoms caused by both conditions are similar. The patient""s medical history may be of no help, or the patient may be incapable of giving a medical history due to age or a language barrier.
To make a reliable diagnosis of obstructive lung disease or restrictive lung disease, physicians often employ a spirometer. A spirometer is a device that measures the flow and volume of air breathed in and out. The patient breathes into the device at the direction of a health professional. The measurements recorded in a spirogram can be used to distinguish obstructive lung disease from restrictive lung disease.
There are, however, drawbacks to spirometry. First, spirometers are rarely available to health professionals treating a patient away from a hospital. Many emergency medical professionals are not trained in spirometry. Getting the patient to a spirometer and to a health professional trained in spirometry often takes time, and the patient""s need for treatment may be urgent. Breathing difficulties can be life-threatening if not diagnosed accurately and treated promptly.
Second, a proper spirogram requires the patient to exert effort to follow the directions of the health professional, such as directions to inhale as much air as possible, to exhale as hard as possible and to expel as much breath as possible. Patients that are short of breath may be incapable of following the directions. Young children also have difficulty with the effort-dependent system.
Because obstructive lung disease and restrictive lung disease are treated with different methods and different medicines, distinguishing the conditions is important. Risks associated with making an incorrect diagnosis are dire. A patient who suffers from congestive heart failure but is misdiagnosed as suffering from chronic obstructive pulmonary disease, for example, may be mistakenly treated with a beta agonist. Beta agonist therapy can significantly increase myocardial oxygen consumption and worsen ischemia for that patient.
In general, the invention is directed to techniques for rapidly and reliably distinguishing obstructive lung disease from restrictive lung disease. In addition, the invention is directed to techniques for monitoring the response of the patient to treatment for the condition.
To distinguish obstructive lung disease from restrictive lung disease, the invention employs measurements of the concentration of carbon dioxide in the breath of the patient. A device such as a capnograph can be used to take these measurements, and the measurements taken by the capnograph are called a capnogram. The capnograph tracks the concentration of carbon dioxide during each exhaled breath.
In a typical capnogram, the carbon dioxide concentration in the breath rises as a patient begins to exhale. The carbon dioxide concentration plateaus, then drops as the patient concludes exhalation. The shape of the curve that follows the carbon dioxide concentration is correlated to the ventilatory status of the patient. In particular, measurements of carbon dioxide concentration can be used to distinguish obstructive lung disease from restrictive lung disease.
In one embodiment, the invention is directed to a method comprising measuring a concentration of carbon dioxide in a breath expired by a patient and using this measurement to determine the presence of obstructive lung disease or restrictive lung disease. The method may take into consideration, for example, the duration of a steady rise of the concentration of carbon dioxide in the breath or the rate of increase of the concentration of carbon dioxide, as measured by the initial angle and slope of the capnogram. The method may also compare the carbon dioxide concentration in the breath with a characteristic curve. The method may further include monitoring the condition of the patient following treatment.
In another embodiment, the invention presents a device comprising a gas sensor that measures the concentration of carbon dioxide in a breath expired by a patient and a processor that determines the presence of obstructive lung disease or restrictive lung disease as a function of the measurement. The device usually includes an output device that reports the determination.
In a further embodiment, the invention presents a method comprising measuring a concentration of carbon dioxide in a breath expired by a patient and guiding treatment as a function of the measurement. Guiding treatment may include determining the presence of lung conditions, determining the severity of the conditions, and selecting medications to treat the conditions.
The invention may provide a number of advantages. For example, the invention quickly provides information to a health professional to guide treatment of the patient. In an exemplary usage, the invention rapidly and reliably distinguishes obstructive lung disease from restrictive lung disease without the need for a spirometer. Moreover, unlike a spirometer, the techniques of the invention may benefit patients that are incapable of following breathing directions. Furthermore, the invention may be small and easily portable, and may be brought to the patient by an emergency medical professional. As a result, the ventilatory status of the patient may be assessed quickly.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.